1. Why do stagecoach wheels turn backwards in movies?
Our eyes can't keep up with the speed of rotation - us seeing the wheel appear to slow down and change direction is our eyes lagging what's really happening.
2. Does a vacuum cleaner work in a vacuum?
Depends if you want to dump stuff into the vacuum or not.
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The scientists at CERN have a backup plan: we give helicopters cancer.
Mad props to XKCD!!!!
(I spent a few years as a graphic artist --not necessarily a good artist-- but usually I detest comics with such bad artwork, but for XKCD I make an exception. It is good.)
Our eyes can't keep up with the speed of rotation - us seeing the wheel appear to slow down and change direction is our eyes lagging what's really happening.
Isn't it also a relic of the "sampling rate" of the film? At something like 24fps it's like strobing a water droplet at the wrong frequency and it looks like its running backwards. Am I completely off on this?
Our eyes can't keep up with the speed of rotation - us seeing the wheel appear to slow down and change direction is our eyes lagging what's really happening.
Not according to What Einstein Told His Barber, by Robert L. Wolke.
Isn't it also a relic of the "sampling rate" of the film? At something like 24fps it's like strobing a water droplet at the wrong frequency and it looks like its running backwards. Am I completely off on this?
True - although I think that's the same thing but with different numbers.
Originally Posted by AV1611VET
Not according to What Einstein Told His Barber, by Robert L. Wolke.
Great. Which was?
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Urban legend is... urban . You made me doubt myself, so I invoke the mighty powers of the Straight Dope. Who weren't too helpful. But thesesources were .
Basically, it's an amorphous solid, in that it has no well-defined melting point. It's molecules are arranged as if they were liquid, but they don't move, like a solid. So it's a solidy liquidy blur.
For all intents and purposes, though, it's a solid. It's hard and shatters when I hit it.
This truly is one of those "great questions". It defies an easy answer.
Here's a link to a discussion of the thermodynamics of glass. (LINKY)
Essentially unlike most crystalline solids in which there is a distinct "melting point" and what is called a first order phase transition as it cools and it doesn't appear to have a "latent heat of fusion" so, many take that to mean it is a supercooled liquid, however there is a second order phase transition which marks other changes in physical features at a certain temperature.
In essence it has a "glass transition temperature", like some polymers. But it goes into an amorphous (ie non-crystalline) phase, but this can affected by the rate of cooling.
The Liquid-->Crystal transition is thermodynamic. It is a change to more energetically stable condition.
The Glass Transition is kinetic. It is not necessarily energetically favorable enough to keep the molecules from continued movement. From what I can gather it isn't a "potential well" where the stable configuration is a hinderance to further molecular movement.
In things like volcanic glasses (obsidian) there are "devitrification" events. You see some crystalline phases form within the glass.
Sounds like this is still a debatable point as to how to correctly classify glass, but it is largely a semantic argument. There are thermodynamic and materials science justifications for a variety of approaches to this question.
As for the ancient window glass. Apparently it has not flowed. That is, as you guys have said, a relic of the glassmaking art of the time (Crown Glass method which winds up with a "spinning step" resulting in thicker bottoms).
Today we apparently use a "float glass" technique which generates nice flat panes.
Summary: You can ask a physicist anything but it takes a chemist to really muck up the waters!
Is their a simple way to explain spin of sub atomic particles ?
Not according to everyday experience as such - spin is a uniquely and completely quantum effect. Often it applies to things with no apparent internal structure like electrons.*
*That said, I know of a few experiments that would eventually lead to an investigation of whether this was really the case.
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I'll see if I can explain this w/o quoting the book itself.
It has to do with camera speed.
Say you had a wheel with only one spoke (humor me here for a sec).
The camera snaps when the spoke is at 12:00, then again when the spoke advances to 11:00, 10:00, etc.
Creating the optical illusion that the wheel is turning backwards.
Ok, thanks. Maybe I didn't describe it well it enough but as I said to thaum, that does sound like the same thing as the eye but with different numbers involved (i.e. a camera's sample rate isn't going to be the same as that of the human eye per se).
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